Die attach adhesives are used to bond the semiconductor device such as a silicon die or chip to a substrate such as a lead frame or a printed circuit board. During assembly of the semiconductor package, the die attach adhesive holds the device firmly in place during wire bonding and encapsulation. They can provide electrical and/or thermal contact between the device and the substrate by means of incorporating electrically and/or thermally conductive fillers in the adhesive formulation.
Examples of commonly used die attach adhesives are eutectic solders, conductive epoxies, and conductive polyimides. Eutectic solders are metal alloys typically made with gold. A "preform", which is a metal foil cut to the shape and size of the semiconductor chip, is deposited on the desired substrate of the package and is heated to a temperature near the melting point of the preform. The chip can then be placed onto the preform with a scrubbing motion. The eutectic solders are expensive and difficult to process. Conductive epoxies are typically low viscosity pastes containing electrically conductive fillers. The epoxy is applied to the substrate by conventional means and the device is then placed in contact with the coated substrate. The epoxy can then be cured in one step. Conductive polyimides are similar to conductive epoxies.
Thermosetting die attach adhesives, primarily epoxy-based formulations are used to bond semiconductor devices to substrates. Rework processes to replace defective chips usually involve the use of heat both beneath the substrate and on top of the device accompanied by a shearing force to remove the die from the substrate. Historically, rework has been relatively simple since the devices are small and widely spaced from each other. However, emerging packaging technologies such as multichip modules use both larger devices and smaller spacing between them and therefore raise the possibility of damage to the substrate during rework. Thus, it is desirable to provide a die attach adhesive composition which allows the reworking to be more readily processable.